Aquaculture produced fish, such as catfish, are susceptible to developing an undesirable off-flavor that causes unpalatability and delays commercial production. Off-flavor is often considered the most significant problem within the catfish industry since, at certain times of the year, over 75% of all production ponds may contain fish that are not marketable due to unacceptable flavor quality. Off-flavor is the result of absorption by fish of hydrophobic terpenoids which are produced by cyanobacteria, Oscillatoria species (cyanobacteria are commonly referred to as blue-green algae). Geosmin (trans-1,10-dimethyl-trans-9-decalol) and 2-methylisoborneol (MIB) are two of the known hydrophobic terpenoids produced by cyanobacteria. Other hydrophobic terpenoids may be produced by blue-green algae, either directly as metabolites or as secondary products from metabolites including 2-isopropyl-2-methyloxypyrazine.
Low levels of these terpenoids accumulate in the tissues of catfish, resulting in an undesirable earthy, musty, or muddy flavor. High levels of these terpenoids give fish a turpentine flavor. In addition to catfish, these same terpenoids likewise affect other aquaculture products.
Identification of the specific chemical agents responsible for the earthy/musty off-flavors became possible with the development of analytical separation techniques. In the mid-1960s, gas chromatography was used to identify two terpenoids, geosmin and MIB, as the most common earthy/musty compounds found in cultures of actinomycetes (Gerber, N. N. and Lechevalier, H. A., 1965, Applied Microbiology 13: 935-938; Gerber, N. N., 1968, Tetrahedron Letters 25: 2971-2974; Gerber, N. N., 1969, Journal of Antibiotics 22: 508-509). These two compounds would later be implicated as the most common causes of flavor problems in aquaculture.
Aschner, M. et al. provided one of the first comprehensive accounts of earthy/musty off-flavors in fish produced in aquaculture (Aschner, M. et al., 1969, Bamidgeh 19: 23-25). They reported that the cyanobacterium Oscillatoria tenuis was associated with earthy off-flavors in common carp grown in ponds in Israel. They suggested that fish acquired the off-flavor either by absorption of an unidentified odorous compound from the water or by consumption of the cyanobacteria masses. They noted that the flavor problem could be alleviated by killing the cyanobacteria with an algicide or by transferring off-flavored fish to clean water. Subsequently, earthy/musty off-flavors were reported in channel catfish grown in ponds in Alabama (Lovell, R. T. and Sackey, L. A., 1973, Trans. Amer. Fish. Soc. 4: 774-777) and in Kansas (Maligalig, L. L. et al., 1973, Food Product Development 7: 86-92). Geosmin produced by cyanobacteria was later found to be the major cause of the off-flavor in Alabama, with MIB also adversely affecting flavor (Lovell, R. T., et al., 1986, Transactions of the American Fisheries Society 115: 485-489). Off-flavors are also common in pond-raised channel catfish in northwest Mississippi; however, MIB, rather that geosmin, has been reported as the primary cause of off-flavor in Mississippi catfish (Martin, J. F., et al., 1988a, Water Science Technology 20: 99-105; Martin, J. F., et al., 1988b, Aquaculture and Fisheries Management 19: 151-157; Van der Ploeg, M., et al., 1992, Water Science Technology 25: 283-290). Martin, J. F. et al. also identified a planktonic cyanobacterium—strongly resembling and tentatively identified as Oscillatoria cf. chalybea, which has been implicated in taste and odor problems in drinking water reservoirs—as the producer of MIB in Mississippi catfish ponds (Martin, J. F., et al., 1991, Water Research 25: 1447-1451).
Earthy/musty off-flavors exist in aquaculture products other than channel catfish raised in the United States. It appears that off-flavors may occur in any species of fish or shellfish raised in most of the commonly-used aquaculture systems. For example, earthy/musty off-flavors have been reported from such widely-different species and geographical regions as rainbow trout raised in freshwater and saline lakes in central and western Canada (Yurkowski, M. and Tabachek, J. L., 1974, Journal Fisheries Research Board of Canada 31: 1851-1858; Tabachek, J. L. and Yurkowski, M., 1976, Journal Fisheries Research Board of Canada 33: 25-35), common carp from fish ponds in Israel (Aschner, M. et al., 1969, Bamidgeh 19: 23-25), wild and farmed Atlantic salmon from locations in Northern Ireland, the Republic of Ireland, and Scotland (Farmer, L. J., et al., 1995, Water Science Technology 31: 259-264), and tilapia from indoor and outdoor pools in the northeastern United States (McLarney, W. O., 1984, The Freshwater Aquaculture Book, Hartley & Marks, Publishers, Point Roberts, Wash.).
Cyanobacteria are responsible for most of the off-flavors encountered in aquaculture. Therefore, preventative measures usually focus on management toward elimination of that group of organisms. Copper-based algicides have long been used in supplies of drinking water and are currently the most commonly used algae-control agents approved for use in aquaculture. However, a relatively small margin of safety exists between concentrations of copper that are toxic to algae but are still safe for the aquaculture product being produced, such as catfish. Moreover, the toxicity of copper to all aquatic organisms is strongly influenced by complex interactions with environmental variables such as pH, water temperature, and concentrations of calcium and dissolved organic matter. These largely unknown interactions have made consistently safe and effective copper treatments for algae control almost impossible. As a result, acceptable copper levels have been based mainly on trial and error. Additionally, the United States Environmental Protection Agency may shortly restrict copper use in aquatic systems in the U.S.
The terpenoids produced by cyanobacteria are released into the water and absorbed mainly through the gills of fish. The chemicals are then absorbed by the lipid-rich tissues of fish, thereby causing the undesired off-flavor. The length of time that the fish remain off-flavor depends on temperature, the concentration of off-flavor chemicals in the water, and the amount of fatty tissue in the fish. For channel catfish, the sensory threshold is estimated to be 8 μg/kg and 0.7 μg/kg for geosmin and MIB, respectively. At warmer temperatures, fish absorb off-flavor compounds more rapidly than at cooler temperatures but also eliminate them faster. Most off-flavor eventually disappears, but off-flavor episodes may last from months to years.
A previous approach (U.S. Pat. No. 4,398,937) uses selective algaecides to control cyanochloronta (blue-green algae) in various bodies of water and, consequently, to control the associated earthy, musty flavors in fish and other cultured organisms. Another approach to the off-flavor problem involves transferring off-flavor fish to flowing, charcoal-filtered water, the results of which showed improved flavor after 3 days and total flavor restoration in 10 days (Lovell, R. T. and Sackey, L. A., 1973, Trans. Amer. Fish. Soc. 4: 774-777). This approach is expensive both in terms of the time and the cost involved, and the charcoal has a limited capacity to adsorb the undesired terpenoids and thus must be replaced fairly frequently. Adsorption is a surface phenomena in which the material taken up is distributed over the surface of the adsorbent, whereas in absorption the material to be taken up is distributed throughout the body of the absorbent. Large volumes of bone charcoal are difficult to obtain and must be recharged using saltwater.
Yet another approach to the problem (U.S. Pat. No. 4,926,795) involves eliminating off-flavor in catfish by treating the aquatic environment with inorganic peroxide additives. This approach claims to achieve flavor reinstatement within 72 hours of treatment.
Another previous approach (U.S. Pat. No. 6,063,287) to reduce the amount of geosmin and MIB in drinking water uses cyclodextrins to sorb the hydrophobic terpenoid. The hydrophobic terpenoid, bounded with cyclodextrin, is then separated from the drinking water. This approach requires the terpenoid-bound cyclodextrin to be separated from the water, requires use of cyclodextrins that are relatively expensive, and would be costly to deploy on a catfish pond or other aquaculture system.
Yet another approach to reducing or eliminating the off-flavor problem in water or aquatic life living in the water (U.S. Pat. No. 6,322,782) relates to treating the water with bioherbicides, specifically the pathogenic bacterium Bacterium NRRLB-30043 (SG-3), for controlling cyanobacteria or algae. The bacterium is used in this approach in an amount sufficient to kill, damage, eliminate, or suppress, and thereby control, the cyanobacteria and/or algae. This approach uses a bacterium that is sensitive to external conditions and is temperamental when in a natural environmental setting.
Although known methods exist to control or reduce off-flavor in aquaculture products, no current technology has proven to be commercially cost effective and efficient. Additionally, introducing bioactive chemical agents such as copper salts to reservoirs or fisheries to control or eliminate algae and off-flavor may be undesirable due to concerns of cost, toxicity, and environmental effects. It remains a significant goal of those skilled in the art to devise a method that controls off-flavor in water and aquaculture products in a cost-effective, environmentally-friendly manner.
The present inventors have designed a method of controlling or eliminating off-flavor in water, fish, and other aquaculture products as a preferred alternative approach that uses no biocides which would linger in the environment or in the fish or that humans will ultimately consume, and thus is environmentally benign.